The present invention relates to the bonding of textile reinforcements to high temperature resistant rubber and, more particularly, this invention relates to an improved adhesive composition for adhering such textile reinforcements to EPDM (ethylene-propylene-diene rubber) for use in reinforced rubber-based products such as high temperature resistant power transmission belts.
With the increased demand for higher temperature power transmission belts, there is a need for an improved adhesive to bond textile reinforcements to rubber compositions used in the manufacture of high temperature resistant articles. Typically, a reinforcing material for use in power transmission belts and other reinforced high temperature resistant rubber products is dipped in a resorcinol-formaldehyde-latex (RFL) suspension to enhance the bonding of the rubber to the material. Typically, the RFL dip is prepared by mixing the RFL prepolymer with a latex. RFL technology is very old chemistry and has been used for many years to attain adhesion to rubber compounds. Most rubber formulations are compounded with phenolic resins containing a formaldehyde donor to continue the crosslinking reaction between the RF in the compound to the RF on the treated textile. The resorcinol-formaldehyde resin provides adhesion to the reinforcing material while the latex provides adhesion to the rubber. Where the surface reactivity of the reinforcing material is low, as in polyester and aramid cords, the cord is usually pre-treated with a composition which improves its reactivity. Compositions that have been used for this purpose include epoxy resins and isocyanates. Conventionally, belts are prepared by bonding a double-twisted cord to the rubber. The cord is immersed in a RFL dip and transported through a vertical oven where it is heated under tension for a predetermined period of time. This causes the cord to elongate and restructure the polyester. This process is known as “stress elongation” or “heat set”. The process dries the adhesive which is coated on the cord. The common RFL dip systems have also been developed for general purpose rubber compounds such as natural rubber and styrene-butadiene rubber. For these rubbers, the latex used in the RFL dip is often a terpolymer based on styrene, butadiene and vinyl pyridine. For less reactive rubber compounds, such as EPDM, the common RFL dip systems are not suitable.
Prior art methods of adhering polyester cord, e.g., to rubbers such as chlorosulfonated polyethylene (CSM) have included the use of neoprene latex and vinyl pyridine latex in the RFL latex dip.
U.S. Pat. No. 3,325,333 to Kigane et al. teaches a method of adhering a polyester cord to a CSM rubber compound by treating the cord with an aqueous adhesive composition containing methylolated blocked organic polyisocyanate and a vulcanizable organic polymer at conventional polyester treating temperatures.
U.S. Pat. No. 3,060,078 to Atwell teaches a method of bonding polyester cord fibers to a CSM rubber compound by treating the cord with a resorcinol-formaldehyde-neoprene latex adhesive composition prior to heat setting the treated cord with the CSM rubber compound.
Japanese (Kokai) Pat. Appln. No. H4-81476 to Toyoda Gosei Co. Ltd. teaches a very specific adhesive obtained by mixing (i) a butadiene rubber latex with (ii) a styrene-butadiene-vinyl pyridine terpolymer latex and (iii) a chlorosulfonated polyethylene latex in weight ratios of the components i/ii=80/20 to 70/30 and the components (i+ii)/iii=95/5 to 75/25 to obtain a rubber latex which is then mixed with a resorcin-formaldehyde resin to provide an adhesive suitable for bonding ethylene-propylene-based rubber to synthetic fibers.
UK Pat. Appln. No. GB 2,042,563 to Neubert describes an aqueous alkaline dispersion of (a) a rubbery vinyl pyridine copolymer latex, (b) a rubbery polybutadiene latex or a rubbery copolymer of at least 80% butadiene, and the balance a mono-ethylenically unsaturated monomer other than vinyl pyridine, e.g., maleic acid or maleic anhydride and (c) a water soluble, thermosetting phenolic-aldehyde resin for bonding glass fibers to rubber.
Another conventional treatment for polyester cord is taught in U.S. Pat. Nos. 5,654,099 and 5,807,634, wherein the polyester cord receives an initial polyphenylisocyanate treatment at 8 to 15 pounds of tension which is heat activated at a temperature in the range of 300° F. to 400° F. for 120 seconds to react the functional groups of the polyphenylisocyanate with the open bond sites in the cord. A RFL adhesive is coated over the reacted polyphenylisocyanate and dried, preferably in an oven, for 120 seconds at a temperature in the range of 180° F. to 275° F. to evaporate the water from the RFL and to keep the RFL from blistering prior to the heat setting of the cord. The heat setting of the polyester cord at 460° F. for 60 seconds takes place after the adhesive is applied and therefore, the adhesive is subjected to this elevated temperature. Adhesion between RFL treated textiles and EPDM rubbers can be achieved if the EPDM rubber is compounded in such a way as to allow for proper reaction chemistry (crosslinking) to occur at the interface of the rubber and the adhesive. Typically, these modifications to the EPDM rubber reduce the desired physical properties and dynamic flex life of the rubber compound. Accordingly, there is a need for an improved adhesive that is able to adhere textiles to an EPDM rubber that is compounded for extended belt life and temperature resistance.